1. Technical Field of the Invention
This invention relates to a while light emission diode useful as back light for various devices such as illuminator, indicator, and display.
2. Description of the Related Art
FIG. 5 in the accompanying drawings is a sectional view showing the white light emission diode of prior art which has been practically used.
This emission diode of prior art comprises GaN-based blue excitation element 11 having n-type semiconductor (n-type GaN) and p-type semiconductor (p-type GaN) grown on sapphire substrate and then die-bonded to metal stem 12.
The blue excitation element 11 has its n-type layer electrically connected to the metal stem 12 and its p-type layer electrically connected to a metal stem 13 via gold wire 14, 15, respectively. The blue excitation element 11 is coated with a predetermined thickness of fluorescent material 16.
The blue excitation element 11, the gold wire 14, 15, the fluorescent material 16 and the metal stems 12, 13 are partially surrounded by transparent molded epoxy resin 17 to form the desired white light emission diode.
In the case of the above-mentioned white light emission diode of prior art, the blue excitation element 11 functions as a blue light source emitting blue light which is then wavelength-converted by the fluorescent material 16 to white light.
FIG. 6 in the accompanying drawings illustrates emission spectrum presented by said white light emission diode of prior art.
As will be understood from this emission spectrum, white light emitted from said white light emission diode of prior art is a combination of the light (in the vicinity of 450 nm) emitted from the blue excitation element 11 and the light (in the vicinity of 580 nm) emitted from the fluorescent material 16.
In other words, this white light emission diode of prior art is not adapted to generate the desire white light from three primary colors, i.e., blue (having the wavelength of 450 nm), green (having the wavelength of 525 nm) and red (having the wavelength of 660 nm). With a disadvantageous consequence, if it is intended to replace the fluorescent lamp conventionally used as indoor lamp or back light for liquid crystal display by such light emission diode, this replacing emission diode will be accompanied with various problem such as a color rendering.
As one of the measures to solve such problems, a white light emission diode has already been proposed, in which the UV rays emitted from ZnO- or GaN-based UV excitation element is wavelength-converted using fluorescent material to white light.
However, such white light emission diode using the UV exciting element as the light source is disadvantageous in that the molded epoxy resin is deteriorated under the effect of the UV rays emitted from the exciting element and a transmission of the molded resin, therefore, a white light emitting output is correspondingly deteriorated as the time elapses.
In view of the situation as has been described above, it is a principal object of this invention to provide a white light emission diode using the UV exciting element as the excitation light source improved so as to present a high color rendering effect when used as the back light for liquid crystal display and to ensure the effective output of light emission for a relatively long period without any significant deterioration of the molded resin.
The object set forth above is achieved, according to this invention, by a white light emission diode comprising an exciting element using a p-type semiconductor and a n-type semiconductor to emit UV rays, a wavelength converting element converting said UV rays emitted from said exciting element to white wavelength light rays by means of fluorescent material and transparent molded resin covering these exciting element and wavelength converting element, said white light emission diode being characterized by that said exciting element is provided on its one surface extending in parallel to a region in which said p-type semiconductor and n-type semiconductor are bonded together with a wavelength converting element and further provided on a side surface of said exciting element lying in the vicinity of said region with insulating film being transparent in the UV wavelength band and in the vicinity thereof, and a p-type ohmic electrode and a n-type ohmic electrode both exhibiting a high flexibility in the UV wavelength band and in the vicinity thereof are arranged to cover the surface of said exciting element other than said one surface.
With the white light emission diode, the exciting element functions as a light source emitting UV rays.
The UV rays emitted toward one side of the exciting element are wavelength converted by the wavelength converting element to white light which is then irradiated outward through the molded resin.
The UV rays emitted toward the surface side of the exciting element other than said one side are reflected on the p-type ohmic electrode and the n-ohmic electrode both having a high reflectivity and do not enter the molded resin.
Consequently, the UV rays are converted by the fluorescent material of the wavelength converting element to white light. In this way, the white light emission diode it presents a high color rendering effect.
Deterioration of the molded resin is minimized and the white light emission diode according to this invention can be used for a long period without significant decrease of its light emission output since none of the UV rays enters the molded resin.
The exciting element has its side surface sloped so that one side has a relatively wide dimension and the side opposed thereto has a relatively narrow dimension, resulting in an improvement of the light emission output.
More specifically, the exciting element is provided on its side surface with the ohmic electrodes having a high reflectivity in the V wavelength band and said side surface is sloped so that the UV rays are reflected on said ohmic electrodes toward the wavelength converting element.
With a consequence, the UV rays to be wavelength converted by the wavelength converting element to the white light increase and the white light emission output correspondingly increases.